An etalon is a pair of mirrors, e.g. dielectric stacks, facing one another, aligned in parallel to one another, and separated by an optical path. At least one of the mirrors is partially reflective. Once light gets into an etalon through one of its partially reflective mirrors, it is temporarily restrained in the etalon's cavity; that is, it bounces back and forth between the two mirror faces for a while. Certain light frequencies are restrained longer than others. The light frequencies that are restrained the longest are called the resonant frequencies. These resonant frequencies occur with periodicity, or near periodicity, according to the equation                     v        =                                                                     ⁢            Mc                                2            ⁢            nL            ⁢                                                   ⁢            cos            ⁢                                                   ⁢            θ                                              (        1        )            where ν is the resonant frequency, M (an integer) is the mode number of a particular resonance point, c is the speed of light, n is the refractive index of the etalon's cavity medium, L is the etalon cavity's physical thickness and θ is the beam angle in the etalon.
Under ideal conditions, integer steps in M will produce periodic intervals in ν. However, under real conditions, n is often a function of frequency due to material dispersion in the etalon's cavity medium. Moreover, frequency dependence of n causes frequency dependence in θ if n diverges from the refractive index outside the etalon. These frequency dependencies of n and θ mean that integer steps in M will not produce precise periodic intervals in ν, but instead will produce non-periodic intervals in ν.
Frequency dependencies caused by material dispersion are generally undesirable. In certain applications, it may be desirable to have precise periodicity in ν. For example, in certain telecommunications applications it may be desirable to keep an etalon's resonance points on a regular grid in order to reverse chromatic dispersion on regularly-spaced channels. This creates a need for corrective tuning to reverse any non-periodicity induced by material dispersion. In other applications, it may be desirable to have quasi-periodicity, that is, to place the etalon's resonance points on an irregular grid. Even there, however, any non-periodicity induced by material dispersion is unlikely to match the desired quasi-periodicity, and corrective tuning is therefore required.